This application is filed pursuant to 35 U.S.C. xc2xa7371 from PCT/EP97/02931, filed Jun. 6, 1997, which in turn claim priority to German application 196 23 706.8, filed Jun. 14, 1996.
Not Applicable.
1. Field of the Invention
The invention relates to a process for preparing linear or branched sulfur-containing polymers, such as polyarylene sulfides, in particular polyphenylene sulfide (PPS), in which the monomers are added to a polymerized, or partially polymerized, mixture.
2. Description of the Related Art
U.S. Pat. No. 4,910,294 describes a process for preparing PPS. The monomers used are dihalogenated aromatic hydrocarbons, in particular dichlorobenzene (DCB), and sodium sulfide, which are reacted in a high-boiling dipolar aprotic solvent, such as N-methylpyrrolidone (NMP). Substantial amounts of solvent must be employed in order to bring the reaction partners to reaction, since the metal sulfide and the aromatic dihalogen compound reaction components are not miscible with one another. At least 3.5 mole of NMP per mole of sulfide are typically employed.
EP-A1-536684 describes the preparation of polyphenylene sulfide in NMP in concentrations higher than 3.5 mol of NMP per mole of sulfide, by adding p-dichlorobenzene to a dehydrated mixture of sodium sulfide and NMP and then polymerizing the mixture. However, a reaction temperature of 280xc2x0 C. is required, and reproducibly high molecular weights, characterized by the melt viscosity of the products, are not achieved.
DE-A1-237 110 describes a process for preparing low-molecular-weight polyarylene sulfide by precharging an alkali metal sulfide in a high boiling polar N-alkyl-2-pyrrolidone and then metering in an aromatic dihalide. Here, the molar ratio of solvent to the sulfide employed reaches at least 3.1. The reaction time is in total 12 h after the start of dihalide addition, at at least 255xc2x0 C.
EP-B1-215 259 describes a process for preparing polyarylene sulfides from dihaloaromatic compounds and alkali metal sulfide in N-methylcaprolactam in which half of the reaction mixture is precharged under gentle reflux. The second half of the mixture, that is including the solvent, is then metered in. The minimum ratioachieved of solvent to the sulfide employed is 3.25. The mean reaction time in the stirred vessel cascade is 12.5 h.
EP 0 374 462 describes the preparation of polyarylene sulfides by continuous addition of sulfide to a mixture of dihaloaromatic compound and polar solvent. The molar ratio of polar solvent to sulfide is from about 1.1 to 1.8:1. The reaction times are from about 10 to 15 hours.
If less solvent is employed in the processes of the prior art, molecular weight and polymer yield are poor; at higher solvent concentrations, the space-time yield is unsatisfactory. To achieve satisfactory space-time yields, the reaction temperature is frequently raised, but this leads to an increase in side-reactions.
The object is therefore to prepare sulfur-containing polymers, in particular polyarylene sulfides, over a wide molecular weight range of molar mass (e.g. Mw=10,000-200,000 g/mol), with good space-time yield, using the mildest possible reaction conditions, and with the least possible contamination by by-products.
It has been found that it is possible to prepare sulfur-containing polymers, in particular polyarylene sulfides, with high space-time yield, with a total solvent requirement of less than 300 g of solvent per mole of sulfide employed, at reaction temperatures not higher than 250xc2x0 C., and with short reaction times (less than 5 hours), if a prepolymer is firstly formed from aromatic dihalogen compound and sulfide in a polar solvent, and is then converted, by addition of aromatic dihalogen compound and sulfide, to a polymer of higher molecular weight.
The invention therefore relates to a process for preparing sulfur-containing polymers from at least one aromatic dihalogen compound and at least one sulfide, in a solvent, where a) a mixture of aromatic dihalogen compound and sulfide is polymerized, b) aromatic dihalogen compound and sulfide are added to the polymerized mixture and c) the reaction mixture is polymerized further.
Sulfur-containing polymers are polymers which contain arylene sulfide units. The arylene constituents of the arylene sulfide units contain mono- or polynuclear aromatics or compound aromatics. The aromatic compounds may also contain heteroatoms. Examples of such aromatic compounds, which may be substituted or unsubstituted, are benzene, pyridine, biphenyl, naphthalene and phenanthrene. Examples of substituents are C1-C6-alkyl, C1-C6-alkoxy, carboxyl, amino and sulfonic acid groups. Examples of compound aromatics are biphenyl and aromatics bonded by ether bridges (arylene ethers).
Preferred sulfur-containing polymers are polyarylene sulfides, in particular polyphenylene sulfide.
Both inorganic and organic sulfides are suitable as sulfide for preparing the polymers. Inorganic sulfides are sulfides of alkali metals and alkaline earth metals, such as lithium sulfide, potassium sulfide, calcium sulfide, and preferably sodium sulfide. The corresponding bisulfides, or hydrogen sulfide, may also be employed, if desired together with alkali metal hydroxides. Suitable organic sulfides are salt-like sulfides with organic cations. For the purpose of the invention, organic sulfides are also those organic sulfur compounds which liberate sulfide anions or bisulfide anions under the conditions of the reaction, for example thioacetamide and thio-N-methylpyrrolidone. Sulfides may also be employed with water of crystallization.
Suitable aromatic dihalogen compounds are dihalogenated aromatic hydrocarbons, inter alia dihalobenzenes, such as o-, m- and p-dichiorobenzene, substituted dihalobenzenes, such as 2,5-dichlorotoluene, 3,5-dichlorobenzoic acid, 2,5-dichlorobenzenesulfonic acid and 3,5-dichlorobenzenesulfonic acid and their salts. Dihalonaphthalenes, such as 1,4-dibromonaphthalene and dihalodiphenyl ethers, such as 4,4xe2x80x2-dichlorodiphenyl ether, may, however, also be employed. Mixtures of different arylene dihalides may likewise be employed. Small amounts (from 0.2 to 5 mol percent based on dihaloaromatic compound) of polyhalogenated aromatic hydrocarbons may also be employed in order to obtain branched or crosslinked sulfur-containing polymers.
Dihaloaromatic compounds and sulfide are also referred to as monomers.
Suitable solvents for preparing the polymer are dipolar aprotic solvents of the amide type, such as dimethylformamide (DMF), dimethylacetamide (DMAc), N-methylcaprolactam and N-alkylated pyrrolidones, such as N-methylpyrrolidone (NMP), or mixtures thereof. NMP is particularly preferred.
The term prepolymers includes oligomers and polymers containing arylene sulfide units. These products usually have a molar mass, expressed as number average molar mass Mn, in the range from 500 to 20,000 g/mol. They may be linear or branched. It is also possible, by employing substituted dihaloaryl compounds (e.g. 2,5-dichlorotoluene) to prepare substituted prepolymers. Prepolymers with halogen end groups, in particular chlorine end groups, are preferably prepared.
The preparation of the prepolymer in step a) is generally carried out by reacting sulfide with dihalogen compounds in polar solvents, such as dimethyl sulfoxide, dimethylformamide (DMF), dimethyl acetamide (DMAc), N-alkylated lactams, such as N-alkylcaprolactams, e.g. N-methyl caprolactam (NMC), N-alkylpyrrolidones, e.g. N-methylpyrrolidone (NMP) or mixtures of these, under mild reaction conditions, i.e. at temperatures not above 250xc2x0 C. The weight of solvent employed per mole of sulfide here is from 150 g to 1000 g, preferably from 250 g to 600 g. If NMP is employed as solvent, this is from 1.5 to 10 mol of NMP per mole of sulfide, preferably from 2.5 to 6 mol of NMP per mole of sulfide. From 0.9 to 1.5 mol of dihaloaromatic compound is employed per mole of sulfide. From 1.05 to 1.3 mol is particularly preferable, giving halogen-terminated prepolymers. The reaction temperatures are from 120 to 280xc2x0 C., preferably from 190 to 250xc2x0 C. The reaction times in step a) are from 0.1 to 5 h, preferably from 0.2 to 2 h. The conversion to prepolymers, based on sulfide, is from 10 to 98%, preferably from 30 to 95%, and particularly preferably from 50 to 80%.
The further addition of the monomers, in step b), to the prepolymerized mixture comprising solvent, prepolymer, the by-product salt and unconverted monomers, may be carried out in either batch or continuous manner. The aromatic dihalogen compound is pumped into the reactor as melt, or with a little solvent, by a metering pump. The sulfides may be added either in solid form through a valve or in liquid form as a melted hydrate, using a suitable pump. In a preferred embodiment of the invention, a homogeneous mixture of sulfide and solvent is added to the reaction mixture with prepolymer. The amount of solvent added with the monomers is adjusted so that there is a total of not more than 300 g of solvent per mole of sulfide in the reactor after the addition.
The ratio of the weight of the added monomers m1 to the weight m2 initially employed for preparing the prepolymer can vary within wide limits. The weight ratio of m1 to m2 can be from 0.1 to 10, preferably from 0.5 to 5, and particularly from 1 to 3.
In another preferred embodiment of the invention, the monomers are added in such a manner that a virtually stationary concentration of the monomers in the reaction mixture is maintained, i.e. the monomer concentration is held as constant as possible during the reaction. In such a stationary state, the amount of monomer added per unit of time is equal to the amount of monomer consumed in the reaction.
If the sulfide is added with water of crystallization, it is advantageous to remove water from the reaction mixture. This may be done in discrete steps or continuously. Besides water, dihaloaromatic compound, solvent, and some H2S are also removed. It has proven advantageous to remove water from this mixture and to return the remaining components to the reactor.
After the addition of the monomers is complete, the reaction mixture is polymerized to completion with stirring (step c). The reaction conditions can be varied within wide limits. The reaction temperatures may be from 120 to 280xc2x0 C., preferably from 190 to 250xc2x0 C. The reaction times may be from 20 minutes to 20 hours, preferably from 1 to 3 hours. In a preferred embodiment of the invention, the total reaction time is not more than 5 hours, at reaction temperatures not higher than 250xc2x0 C.
All the phases of the preparation of the polymer may be carried out either batch-wise or continuously. The reaction may, for example, be using a stirred vessel cascade or in a tubular reactor or using a combination of both of these.
During the reaction, chemically bonded water of crystallization is usually liberated. It can be advantageous for the work-up to remove some or all of the water which is in the reaction mixture, after the reaction is complete. If desired, acids may be added prior to work-up, to neutralize or slightly acidify the reactor contents. Suitable acids are, for example, acetic acid, hydrochloric acid or carbon dioxide.
The polymer is separated off by simple pressure filtration. Other methods for separating solids from liquids may, however, also be employed, for example centrifuging or decanting. It is also possible to work up the resultant suspension by flash evaporation or spray drying, drawing off solvents and other low-molecular-weight substances as the main constituents in vapor form and giving the polymer and the by-product salt as substantially dry solids mixture.
In the case of pressure filtration, the filter residue is expediently washed with solvent in order to remove residues of mother liquor which adhere to it. This separation gives polymer and salt as a solid, and the mother liquor, which may be reused directly for preparing further polymer. The polymer may be separated from the solid, which may also first be dried, by boiling in water and subsequent filtration.
In a version of the work-up, the hot (from 150 to 240xc2x0 C.) reaction mixture is filtered under pressure, giving only the salt as residue, the polymer being redissolved in the filtrate, from which the polymer may be isolated either by spray drying or by crystallization followed by filtration.
The molecular weights of the polyarylene sulfides may be adjusted to desired values via the stoichiometry of the sulfide and dihaloaromatic compound monomers. The maximum molecular weight is generally achieved when the dihaloaromatic compound is employed in a 2 mol % excess, based on sulfide.
The novel process is described below using the example of the preparation of polyphenylene sulfide (PPS), but without being restricted to this.
Sodium sulfide trihydrate is firstly dissolved by heating in NMP at 180xc2x0 C. in a titanium autoclave. Some of the water of crystallization is then distilled off, until an internal temperature of 190xc2x0 C. has been reached. The contents of the autoclave are heated further, and at a temperature of from 215 to 220xc2x0 C. p-dichlorobenzene (DCB) is metered in. The prepolymer is then formed in a reaction time of from 30 min to 1 h at 230xc2x0 C. After this first reaction phase, both reaction partners are metered in, continuously or little by little, over a period of about 1 h. The molar ratio of the components (sulfide:DCB:NMP) after completed addition is typically 1:1.05:2.8. During the reaction, the internal pressure of the reactor increases, since chemically bonded water is liberated. It has proven advantageous to control the water content of the reaction mixture. One way of achieving this is to limit the internal pressure of the reactor to values between 2 and 6 bar. This may be achieved by partially releasing the gas phase of the reactor into a condenser. After completed addition, stirring of the reaction mixture continues for from 1 to 2 hours at from 230 to 235xc2x0 C. The reaction mixture is cooled and filtered, if desired, after dilution with solvent. The filter residue of PPS and sodium chloride is boiled several times with water, filtered, and then dried.
The melting points of the polyphenylene sulfides are from 270 to 305xc2x0 C., typically from 280 to 295xc2x0 C. The melt viscosity is in the range from 5000 to 500,000 mPas (centiPoise), preferably from 50,000 to 250,000 mPas (centiPoise). The melt viscosity is stable without additives: at 300xc2x0 C., it changes by less than 10% over a period of 1 hour.
The sulfur-containing polymers, such as polyarylene sulfides, in particular polyphenylene sulfide, prepared by the novel process are distinguished by high purity and high quality. A particularly noteworthy property is that the polymers have virtually no odor and no discoloration. The polymers also show good characteristics when subjected to thermal stress.
The invention thus also relates to sulfur-containing polymers, such as polyarylene sulfides, prepared by the novel process. The sulfur-containing polymers prepared according to the invention may be converted into shaped articles by melt extrusion. Films and fibers with good mechanical properties may, however, also be produced.
The novel process has a number of advantages:
The amount of solvent employed is at most 300 g of solvent per mole of sulfide employed, so that a good space-time yield is achieved.
The overall reaction time is less than 5 hours.
The reaction temperature is not above 250xc2x0 C. and may even be substantially lower, suppressing side reactions which are among the causes of toxic impurities in the polymer.
In the overall reaction, the polymer is obtained in a yield of at least 90%, typically 95%, based on the amount of sulfide employed.